1. Field of the Invention
The present invention relates to a motor-vehicle-mounted radar apparatus mounted on a vehicle and used to explore obstacles such as other vehicles traveling around the vehicle, for example, ahead of the vehicle, as targets thus securing safety of driving.
2. Description of the Related Art
Conventionally, motor-vehicle-mounted radar apparatus that explores obstacles of a vehicle in the direction of traveling has been developed. As motor-vehicle-mounted radar apparatus, the FM-CW system is employed in which an exploration wave as a continuous sending wave frequency-modulated using a triangular wave as a modulating wave and a beat component caused by a reflected wave from a target are extracted as a beat signal, and the relative velocity and relative range to the target are obtained based on the frequency of the beat signal. Related arts concerning the FM-CW system radar apparatus are disclosed, for example, in the Japanese Patent Laid-Open No. 111395/1977, the Japanese Patent Laid-Open No. 120549/1995, the Japanese Patent Laid-Open No. 80184/1997 and the Japanese Patent Laid-Open No. 145824/1997. Especially in the Japanese Patent Laid-Open No. 120549/1995, a configuration is disclosed whereby the radiation direction of a beam-shaped radio wave transmitted from motor-vehicle-mounted radar apparatus can be changed in order to correctly explore a target such as a vehicle traveling diagonally ahead for example in curvilinear traveling.
FIG. 16 shows a schematic configuration of conventional motor-vehicle-mounted radar apparatus of the FM-CW system 1. The motor-vehicle-mounted radar apparatus 1 explores a target 2 and transmits a radio wave for exploration from an antenna 3 in order to calculate the range to the target 2 and the relative velocity to the target 2. The antenna 3 receives the reflected radio wave reflected off the target 2. The antenna 3 is formed in a beam shape having a sharp range whose gain is high. Thus it is possible to execute scanning with beam direction changed via a scanning mechanism 4 and to detect the direction of the target 2 from the direction of beam in receiving a reflected signal from the target 2. With the range to and direction of the target 2 obtained, the position of the target 2 can be relatively obtained based on the position of the vehicle.
In the exploration according to FM-CW system, a transmitter circuit 5 is used to give an exploration signal frequency-modulated by a triangular wave to the antenna 3 for transmission, a reflected signal received by the antenna 3 is amplifier and frequency-converted by a receiver circuit 6 and converted to a digital signal by an analog-to-digital (hereinafter referred to as A/D) converter circuit 7, then converted to a frequency component by a fast Fourier Transform (hereinafter referred to as FFT) circuit 8. An object detection circuit 9, based on the range R and the relative velocity V to the target 2 based on the frequency component from the FFT circuit 8.
Each of FIGS. 17A and 17B show a principle in which the object detection circuit shown in FIG. 16 explores the target 2 and calculates the range R and the relative velocity V in accordance with the FM-CW system. From the antenna 3 in FIG. 16, an exploration signal 10 for frequency-modulated continuous wave (CW) is transmitted so that frequencies maybe continuously varied on the triangular wave at a constant variation velocity. An exploration 10 wave reflects off the target 2 and a resulting reflected signal 11 that is received by the antenna 3 is delayed as long as the period corresponding to the range R from the exploration signal 10. This causes difference in frequencies for the exploration signal 10 whose frequency is in variation. The relative velocity V is generated to the target 2. Thus the Doppler shift effect is generated on the reflected signal 11, causing difference in frequency from the exploration signal 10.
In the FM-CW system, as shown in FIG. 17B, variation in frequency caused by the Doppler shift effect is reflected differently on an upbeat signal 12 as a beat signal in the frequency rise section where the frequency shift amount of frequency modulation is increasing, and on a downbeat signal 13 as a beat signal in the frequency drop section where the frequency shift amount of frequency modulation is decreasing. Thus the frequency fub of the upbeat signal 12 and the frequency fdb of the downbeat signal 13 can be represented as the following expressions 1 and 2 using the range frequency fr and the Doppler shit frequency fd that are standard beat signal frequencies.
fub=frxe2x88x92fdxe2x80x83xe2x80x83(1)
fdb=fr+fbxe2x80x83xe2x80x83(2)
Here, the range frequency fr is in proportion to the range R to the target 2 and can be represented by the following expression 3 assuming the frequency shift amplitude of the exploration signal of the FM-CW system 10 as a triangular wave as xcex94f, modulating frequency as a triangular wave fm, and the velocity of light C. The Doppler shift frequency fd can be represented by the following expression 4 assuming the relative velocity to the target 2 as V and the wavelength of the exploration signal as xcex. It is also possible to calculate the range R and the relative velocity V respectively from the range frequency fr and the Doppler shift frequency fd by using the expressions 3 and 4.
xe2x80x83fr=4xc3x97xcex94fxc3x97fmxc3x97R/Cxe2x80x83xe2x80x83(3)
fd=2V/xcexxe2x80x83xe2x80x83(4)
As shown in FIG. 16, related arts concerning the motor-vehicle-mounted radar apparatus that scans the beam direction of the antenna 3 are disclosed, for example, in the Japanese Patent Laid-Open No. 64499/1999, the Japanese Patent Laid-Open No. 72651/1999, the Japanese Patent Laid-Open No. 84001/1999 and the Japanese Patent Laid-Open No. 121053/1999. In the Japanese Patent Laid-Open No. 82673/1996, a configuration whereby the short range and long range are switched over for exploring a target. In the Japanese Patent Laid-Open No. 282220/1998, a related technology is disclosed whereby part of data obtained in radar exploration is used to identify a target for a radar for an airframe. In the Japanese Patent Laid-Open No. 38141/1999, a related art is disclosed whereby a mobile-vehicle-mounted radar is used to recognize an obstacle in a three-dimensional image.
Of the related arts that scan the beam direction of an antenna, for example in the Japanese Patent Laid-Open No. 84001/1999 and the Japanese Patent Laid-Open No. 231053/1999, a philosophy is described that a plurality of explorations are carried out in a single scan period and the target direction is estimated from the peak of the reflected signal level obtained according to the variation in beam direction. In order to upgrade the exploration accuracy of the exploration in the target direction using such a philosophy in the related arts, it is necessary to explore a target in more directions and to increase of the frequency of exploration. Such a method suffers from high load of operation processing so that special hardware for high-speed signal processing is required to process data at a high speed. High-speed signal processing has a problem of heat as well as costs. Smooth operation requires a corresponding circuit scale thus upsizing the system configuration.
A method is also available whereby the limits of angle of exploration in a specific section where a target is present is narrowed. This approach requires a complicated hardware configuration and has few merits in terms of costs.
In recognizing a target, it is necessary to prepare a complicated logic in order to extract a true target in case reflected signals from a guard rail, tunnel, or sound-proof wall is received. In the case of a guard rail, the relative velocity calculated after paring processing in which the frequency in the frequency rise section and the frequency in the frequency drop section according to the FM-CW system are combined does to equal the actual velocity of the vehicle. The target thus appears as a moving object, not a stationary object. The travel amount does not coincide with the value obtained from the relative velocity so that the target is determined as a guard rail based on such information. However, as the frequency of exploration increases, the data update rate is accelerated and travel amount is decreased. Thus the relative velocity obtained from the travel amount is less accurate and makes difficult the comparison of relative velocity.
Further, in the FM-CW system, while a combination of the frequency rise section and the frequency drop section is used to calculate the range and the relative velocity, the Doppler shift effect is large for an object approaching at a high relative velocity so that the difference in frequency is large between the frequency rise section and the frequency drop section. In the case of an approach to a target, the beat signal in the frequency rise section deviates to the lower frequency band and processing is difficult for an extremely low-frequency beat signal thus increasing the minimum range in which a target can be detected, compared with the stationary state. As a result, it is impossible to trace the approaching target and get information on whether the target is approaching or deviating in another direction.
In case targets detected during driving are determined as stationary objects, they include objects that the vehicle can clear or pass through. Conventionally, these objects are not under specific criterion but subject to alarms or deceleration control. While a stationary objects may not be determined as a target but excluded from the target of control for these objects, objects that cannot be cleared or passed through cannot be excluded from the target of control.
An object of the invention is to provide motor-vehicle-mounted radar apparatus that can correctly recognize the position and nature of the target.
In the invention, there is provided a motor-vehicle-mounted radar apparatus that is mounted on a vehicle for exploring targets around the vehicle, characterized in that the apparatus comprises:
an antenna formed to have a high gain in a predetermined beam direction, for transmitting an exploration signal in the beam direction and receiving a reflected signal from a target of the exploration signal,
scanning means for performing a scan that varies the beam direction of the antenna within predetermined limits,
direction detecting means for detecting the beam direction varied by the scanning means,
exploration control means for making control to scan repeatedly in the beam direction of the antenna via the scanning means within the predetermined limits and to explore targets in a plurality of beam directions detected by the direction detecting means, each direction making an angle with each other every time exploration is made, and
target recognizing means for calculating the range to a target based on the reflected signal from the target received by the antenna and the exploration signal transmitted from the antenna and for recognizing the target based on the range and the beam direction of the antenna detected by the direction detecting means.
According to the invention, the motor-vehicle-mounted radar apparatus that is mounted on a vehicle for exploring targets around the vehicle transmits an exploration signal in a predetermined beam direction from an antenna and receives a reflected signal from a target. The beam direction of the antenna is varied within predetermined limits by scanning means in a scan and detected by direction detecting means. Exploration control means makes control to scan repeatedly in the beam direction of the antenna via the scanning means within the predetermined limits and to explore targets in a plurality of beam directions detected by the direction detecting means, each direction making an angle with each other every time exploration is made. Target recognizing means calculates the range to a target based on the reflected signal from the target received by the antenna and the exploration signal transmitted from the antenna. Target recognition is made based on the range and the beam direction of the antenna detected by the direction detecting means. Since a plurality of beam directions that are different from antenna scan to antenna scan is obtained by exploration control means, a combination of exploration results obtained from a plurality of explorations assures as high accuracy in exploration as the results obtained from explorations made in beam directions making a smaller angle with each other. Since an angle between beam directions in explorations during a single scan may be larger than a final angle, the target direction can be determined at an accuracy similar to that in a high-speed processing.
The invention is characterized in that the target recognizing means recognizes a target based on the results of explorations in a plurality of beam directions per scan and recognizes the target based on a combination of exploration results obtained from a plurality of prespecified scans when the number of beam directions of a reflected signal is smaller than a prespecified reference value.
According to the invention, recognition of a target can be made based on exploration results obtained from a plurality of scans in case a sufficient number of exploration results cannot be obtained via a single scan. This can upgrade the target recognition accuracy.
The invention is characterized in that the target recognizing means calculates a Doppler shift frequency from frequency shift amount that accompanies traveling of a target based on exploration results obtained from the plurality of scans and varies reflected signals to be combined for target recognition depending on the calculation results.
According to the invention, it is possible to combine candidate exploration results considering time delay between a plurality of scans in order to perform high-accuracy recognition.
The invention is characterized in that the target recognizing means recognizes reflected signals having frequencies excluded from the combination depending on the exploration results as reflected signals from unwanted reflecting objects, not as reflected signals from a target.
According to the invention, reflected signals having frequencies that cannot be combined among a plurality of exploration results are recognized as reflected signals from unwanted reflecting objects, not as reflected signals from a target. Thus it is possible to avoid burdening the system with load of processing on a target that need not pay attention to and perform processing focused on a target that need to pay attention to.
The invention is characterized in that the target exploration is performed via the FM-CW system and that
the target recognizing means subtracts the Doppler shift component of the velocity of the vehicle from the peak data obtained from reflected signals having frequencies excluded from the combination depending on the exploration results in the frequency rise section and the frequency drop section of the FM-CW system in order to calculate the range and the direction.
According to the invention, the Doppler shift component of the velocity of the vehicle is subtracted in the frequency rise section and the frequency drop section of the FM-CW system in order to calculate the range and the direction even if the exploration results are excluded from the combination for a specific target. This allows the position of a target to be explored efficiently.
The invention is characterized in that the target recognizing means recognizes the calculation results of the range and direction as data on unwanted reflecting objects and obtains the position where unwanted reflecting objects assemble from the range and the direction, and determines the position as a shoulder.
According to the invention, the calculation results based on exploration results that could not be combined are recognized as data on unwanted reflecting objects and the position where unwanted reflecting objects assemble is determined as a shoulder Thus, an object that is on or beyond the shoulder can be excluded from attention in order to reduce processing load.
In the invention, there is provided a motor-vehicle-mounted radar apparatus that is mounted on a vehicle for exploring targets around the vehicle, characterized in that the apparatus comprises:
an antenna formed to have a high gain in a predetermined beam direction, for transmitting an exploration signal in the beam direction and receiving a reflected signal from a target of the exploration signal, and
target recognizing means for calculating the range to a target based on the reflected signal from the target received by the antenna and the exploration signal transmitted from the antenna, for recognizing the target based on the range and the beam direction of the antenna detected by the direction detecting means, and for determining the height of the stationary target depending on whether or not a multipath error has effects on the variation in the reflected signal level within a specific frequency limits caused by the range from the target.
According to the invention, it is determined whether or not the target can be cleared based on the variation in the level of the reflected signal from the target caused by the range from the target. The reflected signal from the target is either a signal directly received by the antenna or a signal reflected off a road surface and received by the antenna. These signals generate a phase difference based on the path difference of the reflected signals thus varying the level of a signal received by the antenna. When the target is low, variation in the level of a reflected signal is small and the possibility of clearing the target is high. Since it is determined whether or not the target can be cleared based on a variation in the level of a reflected signal caused by the range from the target, a secure decision is possible via a simple configuration. A reflected signal from a target on the road surface on which the vehicle is traveling passes through a plurality of paths, i.e., a path directly reaching the antenna and a path once reflected off the road surface and reaching the antenna, and thus received in a state where a phase difference based on the path difference is generated. Phase difference occurs between reflected signals received in such a multipath error. Thus a phase difference close to 180 degrees results in a large signal attenuation. Since such effects caused by a multipath error appear depending on the height of a target, it is possible to properly determine the height of a stationary target based on the effects of multipath error.
The invention is characterized in that the target recognizing means determines that the stationary target can be cleared in case the level of the reflected signal from the target suddenly drops in an approach to the target.
According to the invention, it is determined that the target can be cleared in case the level of the reflected signal from the target suddenly drops in an approach to the target. Since the target that can be cleared has a small height from the road surface and goes beyond the beam direction limits of the antenna when the range to the antenna becomes smaller, the level of the reflected signal suddenly drops. Thus, in case the level of the reflected signal from the target suddenly drops in an approach to the target, it is highly possible that the target is at least lower than the antenna position and thus can be cleared.
The invention is characterized in that the target recognizing means has data indicating the variation in the threshold value of the reflected signal level for the range in advance and determines that the stationary target can be cleared in case the level of the reflected signal from the target has dropped below the threshold value within a predetermined limits of range.
According to the invention, a map is formed indicating the variation in the threshold value of the reflected signal level for the range in advance. Thus, in case the level of the reflected signal in an approach to the target has dropped below the threshold value, it can be determined that the stationary target is low and can be cleared.
In the invention, there is provided a motor-vehicle-mounted radar apparatus that is mounted on a vehicle for exploring targets around the vehicle, characterized in that the apparatus comprises:
an antenna formed to have a high gain in a predetermined beam direction, for transmitting an exploration signal in the beam direction and receiving a reflected signal from a target of the exploration signal, and
target recognizing means for calculating the range to a target based on the reflected signal from the target received by the antenna and the exploration signal transmitted from the antenna, for recognizing the target based on the range and the beam direction of the antenna detected by the direction detecting means, the target recognizing means having data indicating the variation in the threshold value of the reflected signal level for the range in advance and determines that the height of the target based on the range where the reflected signal from the stationary target has dropped below the threshold value.
According to the invention, the range where the reflected signal level drops below the threshold value while the user""s car is approaching the stationary target corresponds to the height of the stationary target. Thus it can be determined that the target is low in case the reflected signal level drops at relatively long range and gets higher as the level drops at relatively short range.
The invention is characterized in that the target recognizing means determines whether or not the stationary target can be cleared based on the state in which the reflected signal level drops as the range gets shorter.
According to the invention, it is determined that the target can be cleared in case the reflected signal level has dropped above a certain extent in an approach to the target from the level at long range. This evades the effects of a variation in the reflected signal level caused by difference of target material.
In the invention, there is provided a motor-vehicle-mounted radar apparatus that is mounted on a vehicle for exploring targets around the vehicle, characterized in that the apparatus comprises:
an antenna formed to have a high gain in a predetermined beam direction, for transmitting an exploration signal in the beam direction and receiving a reflected signal from a target of the exploration signal, and
target recognizing means for calculating the range to a target based on the reflected signal from the target received by the antenna and the exploration signal transmitted from the antenna, for recognizing the target based on the range and the beam direction of the antenna detectedby the direction detecting means, and for determining the height of the stationary target based on the range where the reflected signal level suddenly drops in an approach to the target.
According to the invention, the height of a stationary target is estimated depending on the range where the reflected signal level has dropped below a certain level in an approach to the stationary target compared with the level at long range. When the stationary target is relatively low, the target goes beyond the beam limits from the antenna at relatively long range thus avoiding a drop in the reflected signal level. In case the target is relatively high, the drop in the reflected signal level becomes large at short range. Since the reference level used to evaluate the drop in the reflected signal level is the reflected signal level from the target at long range, effects such as the target material can be reduced to estimate the target height at a high accuracy.
In the invention, there is provided a motor-vehicle-mounted radar apparatus that is mounted on a vehicle for exploring targets around the vehicle, characterized in that the apparatus comprises:
an antenna formed to have a high gain in a predetermined beam direction, for transmitting an exploration signal in the beam direction and receiving a reflected signal from a target of the exploration signal, and
target recognizing means for calculating the range to a target based on the reflected signal from the target received by the antenna and the exploration signal transmitted from the antenna and for recognizing the target based on the range and the beam direction of the antenna detected by the direction detecting means,
the target recognizing means recognizing that the target is not a target having a height to be alerted for a traveling vehicle in case the reflected signal level that is recognized exceeds the prespecified reference while the target recognizing means is recognizing the stationary target at longer range than the predetermined range and the reflected signal level drops considerably as the vehicle approaches the target from the vehicle position at the time of recognition.
According to the invention, it is recognized that the target is not a target having a height to be alerted for a traveling vehicle in case the reflected signal level that is recognized exceeds the prespecified reference while the stationary target at longer range than the predetermined range is recognized and the reflected signal level drops considerably as the vehicle approaches the target from the vehicle position at the time of recognition. For example, in case a reflected signal is received from an object, such as a billboard, a sign, and a two-level crossing, above the road surface on which the vehicle is traveling, the target is within the antenna beam direction limits at a distance but goes beyond the antenna beam direction limits as the vehicle approaches the target and causing the reflected signal level to drop considerably. When such an object as will not interfere with the traveling of the vehicle is detected, the object is recognized as a target not to be alerted. This eliminates control such as unnecessary alert or braking.
The invention is characterized in that the target height is the height of the highest section of the target.
According to the invention, it is possible to determine the possibility of clearing the target from the height of the highest section of the target.
The invention is characterized in that the target recognizing means determines whether or not the target can be cleared based on the determined height.
According to the invention, it is possible to determine whether or not the target can be cleared from the height of the highest section of the target. This assures a correct decision.
The invention is characterized in that the target height is the height of the lowest section of the target.
According to the invention, it is possible to determine the possibility of passing through the target from the height of the lowest section of the target.
The invention is characterized in that the target recognizing means determines whether or not the target can be passed through based on the determined height.
According to the invention, it is possible to determine whether or not the target can be passed through from the height of the lowest section of the target. This assures a correct decision.
The invention is characterized in that the target recognizing means determines the target height in a plurality of sections depending on the range from the target and derives a signal for predetermined alarm and/or braking based on the decision results for each section.
According to the invention, the target height is determined based on the presence/absence of the effects of a multipath error in a plurality of sections in an approach to a stationary target. Thus it is possible to make a plurality of decisions in an approach to the target and to avoid unnecessary alarm or braking in case it is determined that the target can be cleared in an early period.
The invention is characterized in that the target exploration is performed via the FM-CW system and that the target recognizing means uses only data in the frequency drop section to estimate the range and the relative velocity to the target when the target recognizing means determines that the relative velocity to the target is larger than the reference velocity.
According to the invention, it is possible to perform an highly accurate detection of a target that is rapidly approaching.